igb: resolve namespacecheck warning for igb_hash_mc_addr

This patch resolves a warning seen when doing namespace checking via
"make namespacecheck"

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>